Electrodeposition of Platinum Nanoparticles in a Room-Temperature

ARTICLE pubs.acs.org/Langmuir

Electrodeposition of Platinum Nanoparticles in a Room-Temperature Ionic Liquid Da Zhang, Wan Cheng Chang, Takeyoshi Okajima, and Takeo Ohsaka* Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-5, Midori-ku, Yokohama 226-8502, Japan ABSTRACT: The electrochemistry of the [PtCl6]2

[PtCl4]2

Pt redox system on a glassy carbon (GC) electrode in a roomtemperature ionic liquid (RTIL) [i.e., N,N-diethyl-N-methyl-N-(2methoxyethyl)ammonium tetrafluoroborate (DEMEBF4)] has been examined. The two-step four-electron reduction of [PtCl6]2
to Pt, i.e., reduction of [PtCl6]2
to [PtCl4]2
and further reduction of [PtCl4]2
to Pt, occurs separately in this RTIL in contrast to the onestep four-electron reduction of [PtCl6]2
to Pt in aqueous media. The cathodic and anodic peaks corresponding to the [PtCl6]2
/ [PtCl4]2
redox couple were observed at ca.
1.1 and 0.6 V vs a Pt wire quasi-reference electrode, respectively, while those observed at
2.8 and
0.5 V were found to correspond to the [PtCl4]2
/Pt redox couple. The disproportionation reaction of the two-electron reduction product of [PtCl6]2
(i.e., [PtCl4]2
) to [PtCl6]2
and Pt metal was also found to occur significantly. The electrodeposition of Pt nanoparticles could be carried out on a GC electrode in DEMEBF4 containing [PtCl6]2
by holding the potential at
3.5 or
2.0 V. At
3.5 V, the four-electron reduction of [PtCl6]2
to Pt can take place, while at
2.0 V the two-electron reduction of [PtCl6]2
to [PtCl4]2
occurs. The results obtained demonstrate that the electrodeposition of Pt at
3.5 V may occur via a series of reductions of [PtCl6]2
to [PtCl4]2
and further [PtCl4]2
to Pt and at
2.0 V via a disproportionation reaction of [PtCl4]2
to [PtCl6]2
and Pt. Furthermore, the deposition potential of Pt nanoparticles was found to largely influence their size and morphology as well as the relative ratio of Pt(110) and Pt(100) crystalline orientation domains. The sizes of the Pt nanoparticles prepared by holding the electrode potential at
2.0 and
3.5 V are almost the same, in the range of ca. 1
2 nm. These small nanoparticles are “grown” to form bigger particles with different morphologies: In the case of the deposition at
2.0 V, the GC electrode surface is totally, relatively compactly covered with Pt particles of relatively uniform size of ca. 10
50 nm. On the other hand, in the case of the electrodeposition at
3.5 V, small particles of ca. 50
100 nm and the grown-up particles of ca. 100
200 nm cover the GC surface irregularly and coarsely. Interestingly, the Pt nanoparticles prepared by holding the potential at
2.0 and
3.5 V are relatively enriched in Pt(100) and Pt(110) facets, respectively.

’ INTRODUCTION The study of platinum nanoparticles has recently received increasing attention because of their unusual catalytic properties and importance of their applications in extensive chemical and electrochemical reactions, such as in the production of hydrogen from methane, oxygen reduction, and formic acid oxidation.1 Various approaches have been employed for the fabrication of Pt nanoparticles, including the solution-phase method, laser ablation, sonochemical method, and so on.2
4 Compared with them, electrodeposition has proven to be the most effective for arbitrarily controlling the morphology and size of the electrodeposited nanoparticles by suitably choosing the electrolysis conditions.5
9 On the other hand, room-temperature ionic liquids (RTILs) have attracted intensive interest as new possible media for electrodeposition of metals, because RTILs have many advantages, such as a wide electrochemical potential window, high ionic conductivity, and good thermal stability.10
13 RTILs were also demonstrated as a favorable template for preparation of controlled and reproducible chemical nanostructures because a protective shell will be fabricated in which anions coordinate with the prepared metal nanoparticles and cations form hydrogen r 2011 American Chemical Society

bridges with anions.14
19 The electrodeposition of Pt and Pt
Au alloy from RTILs has been reported.20
23 He et al. have found that the catalytic performance and utilization efficiency for methanol oxidation of the Pt film prepared in RTILs would be much higher than that prepared in aqueous solution because the Pt nanoclusters formed in RTILs would be smaller.20 Yu et al. have reported a potential-controllable synthesis and deposition of Pt particles in RTIL.24 In this study, deposition of Pt nanoparticles onto a glassy carbon (GC) electrode will be performed in N,N-diethyl-N-methylN-(2-methoxyethyl)ammonium tetrafluoroborate (DEMEBF4) containing potassium hexachloroplatinate(IV) (K2[PtCl6]) or potassium tetrachloroplatinate(II) (K2[PtCl4]). Cyclic voltammetric measurements were carried out to clarify the electrochemistry of the [PtCl6]2

[PtCl4]2

Pt redox system. It is demonstrated that Pt deposition takes place, depending on the potential applied for the deposition, via a disproportionation reaction of [PtCl4]2
to [PtCl6]2
and Pt, as well as via a series of the reductions of Received: August 1, 2011 Revised: October 8, 2011 Published: October 13, 2011 14662

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Table 1. Summary of the Proposed Reactions, Peak Assignments, and Equation Numbers peak

eq

I

1

[PtIVCl6]2
+ 2e
f [PtIICl4]2
+ 2Cl
(on GC surface)

I0 II

1 2

[PtIVCl6]2
+ 2e
f [PtIICl4]2
+ 2Cl
(on Pt nuclei) [PtIICl4]2
+ 2e
f Pt0 + 4Cl
(on GC surface)

II0

2

[PtIICl4]2
+ 2e
f Pt0 + 4Cl
(on Pt nuclei)

III

3

Pt0 + 4Cl

2e
f [PtIICl4]2


IV

4

[PIICl4]2
+ 2Cl

2e
f [PtIVCl6]2
(on GC surface)

IV0

4

[PIICl4]2
+ 2Cl

2e
f [PtIVCl6]2
(on Pt nuclei)

V

proposed reaction

oxidation (dissolution) of the product(s) originating from the reduction of DEMEBF4 5

2[PtIICl4]2
a [PtIVCl6]2
+ Pt0 + 2Cl


[PtCl6]2
to [PtCl4]2
and further [PtCl4]2
to Pt. Characterization of the prepared Pt nanoparticles was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electrochemical techniques. Oxidation of formic acid at the prepared electrodes was also carried out with a view to exploring the crystalline facet of the deposited Pt particles.

’ EXPERIMENTAL SECTION Reagents. DEMEBF4 was purchased from Kanto Chemical Co., Ltd. K2[PtCl6], K2[PtCl4], and sulfuric acid (Wako Pure Chemical Industries) were of analytical grade and were used as received. All aqueous solutions were prepared with deionized water purified by a Millipore Milli-Q system (Millipore, Japan).

Figure 1. CVs obtained at the GC electrode in DEMEBF4 containing 70 mM K2PtCl6 (solid line) and K2PtCl4 (dashed line). Potential scan range: (A)
2.0 to +1.5 V and (B)
3.5 to +1.5 V vs Pt wire. The insets in (A) and (B) are CVs obtained at the GC electrode in DEMEBF4. Potential scan rate: 500 mV s
1.

Electrodeposition of Pt Particles. All the electrochemical experiments were performed with a three-electrode electrochemical cell containing GC as a working electrode, spiral platinum as an auxiliary electrode, and potassium chloride-saturated silver/silver chloride (Ag| AgCl|KCl(satd)) as a reference electrode or Pt wire as a quasi-reference electrode using an ALS/Chi 750A electrochemical analyzer at 65 ( 1 °C. The potential of the Pt quasi-reference electrode was monitored by dissolving a small amount of ferrocene in DEMEBF4, whose potential was previously evaluated against the Ag|AgCl|KCl(satd) electrode. The potential of the ferrocene in DEMEBF4, which is 0.395 V vs Ag|AgCl| KCl(satd), was 0.17 V vs the Pt wire quasi-reference electrode. In such a way, a potential of ca.
0.225 V vs Ag|AgCl|KCl(satd) could be estimated for the Pt quasi-reference electrode. Prior to each measurement, nitrogen (N2) gas was bubbled directly into the cell solution for at least 15 min to obtain a N2-saturated solution, and during every measurement, N2 gas was flushed over the cell solution. Before electrodeposition of platinum, GC electrodes (3 mm in diameter) were polished on polishing microcloth (Marumoto Struers Kogyo) with alumina powder [particle diameter: 1.0 and 0.06 μm (Marumoto)]/ water slurries. After polishing, the GC electrodes were rinsed, sonicated for 10 min, and stored in deionized water. Platinum deposition onto the GC electrodes was accomplished by holding the potential at
2.0 or
3.5 V vs the Pt wire quasi-reference electrode in N2-saturated

DEMEBF4 containing 70 mM K2PtCl6 or 35 mM K2PtCl4. The electrodeposited Pt on the GC electrodes was electrochemically cleaned by repeatedly scanning the potential between
0.2 and +1.5 V vs Ag| AgCl|KCl(satd) at a potential scan rate of 500 mV s
1 for 10 min in 0.5 M H2SO4 aqueous solution under a N2 gas atmosphere. All the peak assignments for the cyclic voltammograms (CVs) are summarized in Table 1.

Oxidation of Formic Acid on Electrodeposited Pt Particles. The relative ratio of Pt single-crystalline domains [i.e., Pt(111), Pt(110), and Pt(100)] on the surface of Pt particles was examined by measuring the electro-oxidation of formic acid on the Pt particle-deposited GC electrodes in N2-saturated 0.5 M H2SO4 solution containing 0.1 M HCOOH.25
27 The cyclic voltammetric measurements for the hydrogen adsorption/desorption on the Pt particle-deposited GC electrodes were performed in N2-saturated 0.5 M H2SO4 solution by scanning the potential between
0.18 and +0.5 V vs Ag|AgCl|KCl(satd) at a potential scan rate of 50 mV s
1. The real surface area of the Pt particles was estimated from the thus-measured voltammograms by measuring the charge consumed during the hydrogen desorption assuming that 210 μC cm
2 corresponds to a monolayer of adsorbed hydrogen.28 Sample Preparation for TEM and SEM Measurements. A 70 mM concentration of K2PtCl6-containing DEMEBF4 solution was 14663

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prepared by adding K2PtCl6 into DEMEBF4 followed by sonication for 5 h until K2PtCl6 was totally dissolved. In the sample preparation for SEM measurements, Pt particles were deposited onto the GC electrode by keeping the electrode potential at
2.0 or
3.5 V for 480 s in DEMEBF4 containing 70 mM K2PtCl6 without stirring. The Pt particles for the TEM measurements were prepared by the same procedure as used for the preparation of the SEM samples except for the solution was stirred (500 rpm). The Pt particles were collected from the Pt particle-dispersed DEMEBF4 solution under centrifugation (10000 rpm for 30 min), then washed with 99.5% ethanol in an ultrasonic redispersion
centrifugation process two times, and dispersed into 1 mL of ethanol solution. SEM analysis of the deposited Pt nanoparticles was carried out using a Hitachi S4700 field emission scanning electron microscope (Hitachi, Japan), and TEM images were captured by a JEM-2010 field emission transmission electron microscope (JEOL, Japan).

’ RESULTS AND DISCUSSION Redox Behavior of [PtCl6]2
in DEMEBF4. Typical CVs at the

GC electrode in DEMEBF4 are shown as insets in Figure 1. DEMEBF4 will be reduced when the potential is more negative than
3.0 V, and the oxidation peak, V, is observed at ca. 0.7 V, which may correspond to the oxidation of the reduction product of DEME+. Parts A and B of Figure 1 depict typical CVs obtained at the GC electrode in N2-saturated DEMEBF4 containing 70 mM [PtCl6]2
or [PtCl4]2
. From the comparison of these CVs, the cathodic and anodic peaks observed at ca.
1.1 and 0.6
0.7 V, i.e., I and IV, are considered to correspond to the reduction of [PtCl6]2
to [PtCl4]2
and the reverse reaction, respectively (i.e., eqs 1 and 4). ½PtIV Cl6 2
þ 2e
f ½PtII Cl4 2
þ 2Cl


ð1Þ

½PtII Cl4 2
þ 2Cl

2e
f ½PtIV Cl6 2


ð4Þ

When the potential was scanned to
3.5 V in the [PtCl6]2
containing solution, the second cathodic peak, II, was observed at ca.
2.8 V, in agreement with the first cathodic peak observed in the [PtCl4]2
solution (Figure 1B), indicating that this peak corresponds to the reduction of [PtCl4]2
to Pt (eq 2). ½PtII Cl4 2
þ 2e
f Pt0 þ 4Cl


ð2Þ

Figure 2A shows the CVs recorded continuously (five times) for the [PtCl6]2
/[PtCl4]2
redox couple between
2.0 and +1.5 V. After this successive potential cycle, the GC electrode was transferred into a 0.5 M H2SO4 solution for observing the cyclic voltammetric response, but the characteristic peaks of Pt, corresponding to the hydrogen adsorption/desorption and the oxide layer formation and its reduction, were not observed (inset in Figure 2A). The redox response around 0.4 V is considered to be due to the electroactive functional group, such as the quinone moiety on the GC electrode surface.29 On the other hand, the GC electrode at which the potential was cycled between
3.5 and +1.5 V indicated a characteristic CV response expected for the Pt electrode in 0.5 M H2SO4 solution (inset in Figure 2B). Thus, peak II corresponds to the reduction of [PtCl4]2
to Pt on the GC electrode surface. From Figure 2B we can see that after the first scan (from 0.0 to
3.5 V and then to 1.5 V and returning to 0 V) the potential of the cathodic peak (I) corresponding to the reduction of [PtCl6]2
to [PtCl4]2
shifts anodically to ca.
1.0 V (I0 ) and the onset potential of the second reduction is also shifted to ca.
1.4 V (II0 ), while the current of the second

Figure 2. CVs at (A, B) GC and (C) Pt electrodes in DEMEBF4 containing 70 mM K2PtCl6. Potential scan range: (A)
2.0 to +1.5 V, (B)
3.5 to +1.5 V, and (C)
2.0 to +1.5 V vs Pt wire. The insets in (A) and (B) show the CVs of the electrodes, which were obtained after the potential cycle was repeated five times in each case, in N2-saturated 0.5 M H2SO4 solution. Potential scan rate: 500 mV s
1.

reduction peak (II) increases after the first cycle because of the reduction of DEME+ on the deposited Pt particles. The fact that the potentials of the first and second reduction peaks after the second cycle (I0 and II0 ) in Figure 2B are similar to those on the polycrystalline Pt electrode (Figure 2C) supports the above 14664

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Figure 3. CVs obtained at Pt particle-deposited GC electrodes in N2-saturated DEMEBF4 containing 70 mM K2PtCl6. The Pt particle-deposited GC electrodes were prepared by holding the potential of the GC electrode at
2.0 V for (—) 60, ( 3 3 3 ) 300, and (---) 1800 s in N2-saturated DEMEBF4 containing 70 mM K2PtCl6. Inset A was obtained at the Pt particle-deposited GC electrodes (electrolysis time at
2.0 V: (a, —) 60, (b, 3 3 3 ), 300, and (c, ---) 1800 s) in N2-saturated 0.5 M H2SO4 solution. Inset B was obtained at the GC electrodes, which were kept in DEMEBF4 containing 35 mM K2PtCl4 for (a, black line) 1 min, (b, red line) 2 min, (c, green line) 1 h, and (d, blue line) 15 h, in N2-saturated 0.5 M H2SO4 solution. Potential scan rate: 500 mV s
1.

Figure 4. TEM (a, b) and SEM (c, d) images of the Pt particles electrodeposited on GC electrodes. Pt particles were electrodeposited in N2-saturated DEMEBF4 containing 70 mM [PtCl6]2
by holding the potential at (a, c)
2.0 and (b, d)
3.5 V for 480 s.

assignments of peaks I0 and II0 . These results indicate that the formation of Pt on the GC electrode occurs during the first scan and that the two-step reduction of [PtCl6]2
after the second scan may take place at the Pt particles, which were deposited on

the GC electrode during the first scan, with a lower overpotential compared with that at the GC electrode. Recently, much attention has been devoted to the formation of Au particles in RTILs via an electrodeposition of [AuCl4]
to Au 14665

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Langmuir Scheme 1. Schematic Illustration of Pt Deposition via (A) the Disproportionation Reaction of [PtCl4]2
to [PtCl6]2
and Pt and (B) the Four-Electron Electroreduction of [PtCl6]2
to Pt

as well as via a disproportionation of the two-electron reduction product of [AuCl4]
(i.e., [AuCl2]
) to [AuCl4]
and Au, because the two-step reduction of [AuCl4]
to [AuCl2]
and further reduction of [AuCl2]
to Au occurs separately in RTILs in contrast to the one-step electroreduction of [AuCl4]
to Au in aqueous media30 and also because the size and morphology of Au particles and the relative ratio of the Au(111), Au(110), and Au(100) crystalline orientation domains constituting the surface are found to largely depend on whether the Au particles are formed by the three-electron reduction or the disproportionation of the two-electron reduction product ([AuCl2]
) of [AuCl4]
.31 Similarly to the electroreduction of [AuCl4]
, as mentioned above, we have found the two-step reduction of [PtCl6]2
to Pt in DEMEBF4, i.e., the reduction of [PtCl6]2
to [PtCl4]2
and further reduction of [PtCl4]2
to Pt, and thus the formation of Pt particles via a disproportionation of [PtCl4]2
to [PtCl6]2
and Pt can be also expected. In a view to investigating whether the disproportionation reaction of [PtCl4]2
takes place, GC electrodes were polarized at
2.0 V, at which [PtCl4]2
is formed by the two-electron reduction of [PtCl6]2
, for 60, 300, and 1800 s. After that, the electrodes were rinsed in water and then transferred into 0.5 M H2SO4 solution for measuring their cyclic voltammetric response. The obtained cyclic voltammetric response is characteristic of the Pt electrode (inset in Figure 3), and the current intensity increases with increasing electrolysis time. These facts clearly demonstrate the formation of Pt metal via a disproportionation

ARTICLE

of [PtCl4]2
: 2½PtII Cl4 2
a ½PtIV Cl6 2
þ Pt0 þ 2Cl


ð5Þ

Using the thus-prepared Pt-deposited GC electrodes, the cyclic voltammetry measurements were carried out in DEMEBF4 containing 70 mM K2PtCl6 in the potential range of
1.8 to +1.5 V, and the results are shown in Figure 3. As expected, the larger anodic peaks (III and IV0 ) corresponding to the oxidations of Pt to [PtCl4]2
and [PtCl4]2
to [PtCl6]2
and the larger cathodic peak (I0 ) for the reduction of [PtCl6]2
to [PtCl4]2
were obtained at the electrode prepared by the longer electrolysis at
2.0 V. To confirm the formation of Pt metal via the disproportionation of [PtCl4]2
, 35 mM K2PtCl4 was dissolved in DEMEBF4, and the temperature of the solution was kept at 65 °C. After some time (typically 1 and 2 min and 1 and 15 h), the GC electrodes were soaked in the solution with the expectation that Pt particles might be adsorbed on the electrode surface if they are produced and taken out of this solution and washed with H2O, and then the CVs were measured using these electrodes in N2-saturated 0.5 M H2SO4 solution (inset B in Figure 3). The obtained cyclic voltammetric responses are characteristic current
potential curves expected for the Pt electrode (cf. inset A in Figure 3), demonstrating clearly that [PtCl4]2
disproportionates to produce Pt metal as is claimed when the same redox level is achieved by the two-electron electrochemical reduction of [PtCl6]2
(as mentioned above). Recently, Yu et al.24 have examined the formation of Pt particles at an indium tin oxide (ITO) electrode at a high and low overpotential (i.e.,
2.8 and
1.8 V vs Ag/AgCl(KCl(satd)) in 1-N-butyl-3-methylimidazolium hexafluorophosphate containing 20 mM H2PtCl6 and have demonstrated the following remarkable points: When a high overpotential was applied, the nanoparticles preferred to deposit onto the ITO electrode surface since the formation rate of the so-called Pt adatoms [Pt0]ad is fast, and as a result, the stabilization and thus solubilization of [Pt0]ad by ionic liquid (IL) molecules through the steric and/or electrostatic interaction(s) are suppressed. Conversely, when a low overpotential was applied, [Pt0]ad tended to form clusters that are efficiently stabilized by IL molecules through the steric or electrostatic interaction(s) between both components, and the nanoparticles preferred to solubilize into IL solvent to form a homogeneous dispersion. In this case, by taking the potentials used for the deposition of Pt particles into account, as mentioned above, it is thought that the overall four-electron reduction of [PtCl6]2
to Pt metal occurs at a high overpotential of
2.8 V, while at a low overpotential of
1.8 the disproportionation of [PtCl4]2
takes place to form Pt metal and [PtCl6]2
. TEM and SEM Analyses of Electrodeposited Pt Particles. Typical TEM and SEM images of Pt particles electrodeposited by holding the potential at (a)
2.0 and (b)
3.5 V are shown in Figure 4. From the comparison of the TEM images (Figure 4a,b), we can see that the sizes of the Pt nanoparticles prepared by holding the electrode potential at
2.0 and
3.5 V, at which the Pt particles are formed via the disproportionation of [PtCl4]2
to [PtCl6]2
and Pt and the four-electron reduction of [PtCl6]2
to Pt, respectively, are almost the same, in the range of ca. 1
2 nm. These small nanoparticles are “grown” to form bigger particles with different morphologies as seen from the SEM images (Figure 4c,d): In the case of the deposition at
2.0 V, the GC 14666

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Figure 5. (A, B) Oxidation of formic acid at (A) Pt particle-deposited GC electrodes and (B) the polycrystalline Pt electrode in N2-saturated 0.5 M H2SO4 solution containing 0.1 M HCOOH. (C, D) CVs obtained at (C) Pt particle-deposited GC electrodes and (D) the polycrystalline Pt electrode in N2-saturated 0.5 M H2SO4 solution. Pt particle-deposited GC electrodes were prepared in DEMEBF4 containing 35 mM K2PtCl4 by holding the potential at
3.5 V for (a) 1 and (b) 2 min and at
2.0 V for (c) 1 and (d) 2 min. Potential scan rate: 50 mV s
1. Current densities are given on the real surface area scale.

electrode surface is totally, relatively compactly covered with Pt particles of relatively uniform size of ca. 10
50 nm. On the other hand, in the case of the electrodeposition at
3.5 V, small particles of ca. 50
100 nm and the grown-up particles of ca. 100
200 nm cover the GC surface irregularly and coarsely. These different deposition morphologies may originate from the different deposition mechanisms. In the former case, after Pt nuclei are formed on the GC surface, the subsequent deposition of Pt may occur on the bare GC surface rather than on the previously formed Pt nuclei on the basis of the equilibrium of the disproportionation reaction (eq 5). Thus, Pt particles of relatively uniform sizes relatively uniformly and compactly cover the whole of the GC surface (Scheme 1A; see Figure 4c). On the other hand, in the latter case, i.e., in the case of the four-electron electrodeposition of [PtCl6]2
to Pt, after Pt nuclei are formed on the GC surface, the subsequent four-electron reduction of [PtCl6]2
may favorably take place on the previously formed Pt nuclei, because the overpotential for the four-electron reduction of [PtCl6]2
to Pt is lower on the Pt particles than on the GC surface. Thus, the favorable deposition growth of Pt continuously occurs on Pt particles, resulting in morphologically irregularly grown-up Pt particles (Scheme 1B; see Figure 4d).

Oxidation of Formic Acid and Hydrogen Adsorption/ Desorption on Pt Particle-Deposited GC Electrodes. Figure 5A

illustrates the characteristic CVs for the oxidation of formic acid obtained at the GC electrodes modified with Pt nanoparticles by holding the potential at different potentials in DEMEBF4 containing 35 mM K2PtCl4. It should be noted here that the peak current heights (expressed as current densities) of voltammograms c and d are on the whole smaller than those of voltammograms a and b, which results from the fact that the real surface areas of the Pt particles prepared at
2.0 V for 1 (or 2) min are larger than those of the Pt particles prepared at
3.5 V for 1 (or 2) min, in agreement with the expectation from the SEM images of the Pt particle samples prepared at
3.5 and
2.0 V (Figure 4c,d). Two oxidation peaks were observed in the anodic and cathodic potential sweeps. During the anodic sweep, the peak at ca. 0.36 V vs Ag|AgCl|KCl(satd) is ascribed to the direct oxidation of formic acid on the Pt(111) plane, and the formation of a poisoning CO intermediate on it is very slight. The peak at ca. 0.66 V corresponds to the oxidation of formic acid on the Pt(110) and Pt(100) crystalline planes, and the surface is poisoned by the intermediate CO.25,33 During the cathodic sweep, the oxidation of the poisoning intermediate CO on the Pt(110) and Pt(100) planes was observed at ca. 0.54 and 0.23 V, respectively.25,33 14667

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Langmuir We can find that, in voltammograms a and b, the peak currents corresponding to the oxidation of the intermediate CO (at 0.54 V) on Pt(110) are higher than those (at 0.23 V) on Pt(100). This result is similar to that obtained at the polycrystalline Pt electrode (Figure 5B). On the other hand, in voltammograms c and d, the relative peak current heights at 0.23 and 0.54 V are reversed. Thus, it can be speculated that Pt nanoparticles prepared at
2.0 and
3.5 V are relatively enriched in Pt(100) and Pt(110) facets, respectively. In Figure 5C are shown the CVs obtained at the Pt particledeposited GC electrodes in 0.5 M H2SO4 solution. The CV features in the hydrogen adsorption/desorption region could be rationalized on the basis of hydrogen electrochemistry at single-crystalline Pt electrodes in the same acidic solution.25,26,32,33 The H-desorption peaks at ca.
0.10 V can be correlated with the Pt(110) sites.25,26,32,33 Jose et al. found that the signals around 0.34
0.37 V (vs RHE) correspond to the wide Pt(100) terraces, and the peak at 0.27 V (vs RHE) is associated with the Pt(100) step sites. Therefore, the more positive H-desorption peak at ca. 0.03 V suggests the presence of Pt(100) step sites on Pt(111) domains and the Pt(100) ordered domain close to a step or a defect.25,26,32,33 The peak heights at
0.1 V are higher than those at 0.03 V in voltammograms a and b, which is analogous to the result at the polycrystalline Pt electrode (Figure 5D). On the other hand, in voltammograms c and d, the peak heights at 0.03 V are comparable to or larger than those at
0.1 V. These results demonstrate that the Pt particles prepared at
3.5 and
2.0 V are relatively enriched in Pt(110) and Pt(100) facets, respectively, when these facets are compared.

’ CONCLUSIONS Cyclic voltammetric behaviors of the [PtCl6]2
/[PtCl4]2
and [PtCl4]2
/Pt redox couples at a GC electrode in a DEMEBF4 ionic liquid have been investigated for the first time. The electrodeposition of Pt was found to occur, depending on the applied potential, via a disproportionation reaction of [PtCl4]2
to [PtCl6]2
and Pt as well as via a series of reductions of [PtCl6]2
to [PtCl4]2
and further [PtCl4]2
to Pt; e.g., it takes place at
3.5 V via the four-electron reduction of [PtCl6]2
and at
2.0 V via the disproportionation of the two-electron reduction product ([PtCl4]2
) of [PtCl6]2
. Furthermore, the deposition potential of Pt nanoparticles was found to largely influence their size and morphology as well as the relative ratio of Pt(110) and Pt(100) crystalline orientation domains. ’ AUTHOR INFORMATION Corresponding Author

*Phone: +81-45-9245404. Fax: +81-45-9245489. E-mail: ohsaka@ echem.titech.ac.jp.

’ ACKNOWLEDGMENT The present work was financially supported by a Grant-in-Aid for Scientific Research (A) (No. 19206079) to T. Ohsaka from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. D.Z. gratefully acknowledges the Government of Japan for a MEXT Scholarship. A scholarship to W.C.C. from the Taiwanese Government is also gratefully acknowledged.

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dx.doi.org/10.1021/la202992m |Langmuir 2011, 27, 14662–14668